1. Field of the Invention
The present invention relates to a plasma polymerizing apparatus, in particular to a plasma polymerizing apparatus having an electrode with a lot of uniform edges. The apparatus is used in surface treatments of metal, polymer, ceramic etc. and is capable of improving usability of substrates by modifying surfaces of the substrates or forming coating films on the surfaces of the substrates.
2. Description of the Prior Art
In general, when a substrate is surface-treated in plasma discharge, a modified layer having high hardness and abrasion resistance is formed on the surface. There are a lot of surface-modified products such as magnetic disks, optical disks and carbide tools. In addition, when a film coated on the surface of a steel plate is treated in plasma discharge, a hardened coating steel plate having high durability and corrosion resistance can be implemented.
PCT WO99/28530 discloses a plasma polymerizing apparatus, as shown in FIG. 1. The apparatus comprises a vacuum chamber 20, an electrode 4 installed inside of the vacuum chamber 20, vacuum pumps 5, 6 for adjusting pressure of the vacuum chamber 20, gauges 7, 8 for measuring vacuum level, a power supply 40 for applying voltage to the electrode 4, and gas adjusting devices 9, 10 for injecting monomer gas and non-monomer gas such as nitrogen around a substrate 4a to be surfaced-modified.
A substrate 4a, which will be modified by plasma discharge, is placed in the chamber 20. The rotary pump 5 is operated in order to keep inner pressure of the chamber 20 as about 10xe2x88x923 Torr, which is confirmed with the thermocouple gauge 7. The diffusion pump 6 is operated in order to keep the inner pressure of the chamber as about 10xe2x88x926 Torr, which is confirmed with the ion gauge 8. The substrate 4a is biased as an anode by the power supply 40, whereas the opposite electrode 4 is grounded. After the inner pressure of the chamber is kept as a certain vacuum, monomer gas and non-monomer gas are orderly injected to the chamber 20. When the inner pressure of the chamber reaches a certain level, the chamber is plasma-discharged by direct current or high frequency. At this time, molecular bonding of the gases is cut off in plasma discharge, and a polymerized layer is formed on the surface of the substrate placed between the electrodes by combining cut bondings with actively positive ions and negative ions.
The conventional plasma polymerizing apparatus comprises an plate-shaped electrode, as shown in FIG. 2. Since the plate-shaped electrode has a certain dimension, the current in DC discharge is concentrated on the edges of the electrode. In this case, the gas to be polymerized can be carbonized easily on the edges of the electrode, on the contrary, not easily on the middle portion of the electrode. When the carbonization is varied in accordance with the each portion of the electrode, the voltage applied to the electrode becomes non-uniform, which makes the discharge in the chamber unstable and results in arc discharge. Accordingly, the plasma-polymerized product cannot have a good quality.
In addition, in a case of continually performing plasma polymerizing processing on band-type metals, the size of the electrode increases in order to improve productivity. As the size of the electrode increases, the difference of the current density on the each portion of the electrode becomes larger, and the uniformity of the material to be polymerized, becomes lower.
It is an object of the present invention to provide an electrode having uniform current density without concentrating of the current density on some parts, which is capable of keeping uniformity and performing high quality plasma deposition in spite of the carbonization.